The "unset" command:

Usage: fossil settings ?SETTING? ?VALUE? ?OPTIONS?
or: fossil unset SETTING ?OPTIONS?
The "settings" command with no arguments lists all settings and their
values. With just a SETTING name it shows the current value of that setting.
With a VALUE argument it changes the property for the current repository.
Settings marked as versionable are overridden by the contents of the
file named .fossil-settings/PROPERTY in the check-out root, if that
file exists.
The "unset" command clears a setting.
Settings can have both a "local" repository-only value and "global" value
that applies to all repositories. The local values are stored in the
"config" table of the repository and the global values are stored in the
$HOME/.fossil file on unix or in the %LOCALAPPDATA%/_fossil file on Windows.
If both a local and a global value exists for a setting, the local value
takes precedence. This command normally operates on the local settings.
Use the --global option to change global settings.
Options:
--global set or unset the given property globally instead of
setting or unsetting it for the open repository only.
--exact only consider exact name matches.
See also: configuration